That is called a Dry-Rot - rubber loses it's oils over time and loses flexibility and therefore the ability to provide the cushion for comfortable drive.
38K is not that much as people drive them well above that. I had Michelins with over 90k kms with similar dry rotted condition and they still would hold up air and wouldn't lose grip in wet. They were hard and noisy and I had one opened up for repairs (minor puncture) and on the inside it was very good condition.
Dry rot is mostly on the outer (tread) surface, there are atleast 3 to 4 internal layers of rubber and mesh for structural integrity..
I would say surface dry rot is not a big deal just keep the air pressure in check (30-32psi) and make sure none of the tyres loses air. If you can live with the noise and don't go over 80 and 100 on highways these should be fine for few trips per month.
Changing tyres is a necessity only when there are safety concerns, major side wall damage, or they are more than 5-year old. But if you have money to buy new then it's your choice, I have seen people change oils in cars within 6 months with 500-1000km mileage. And some drive over 10k kms and change maybe once a year.. it is all good just a matter of peace of mind.
